Not as much as you think. I wouldn't use MDF or OSB but I've seen plenty of people use the plywood. The media loses energy quickly as it leaves the gun and from 24"-36" inches is relatively easy on the back wall. People who use a plywood box heavily will line high wear area's with thin sheet metal, like 29ga picked up at a HVAC place or metal roofing place (metal roofing straight off the coil, unformed). In case other haven't seen it, here's a great cabinet conversion that looks really cool. http://www.hotrodders.com/forum/newb...ew-118463.html I think I've posted that before but in case some haven't seen it. I would have one like that but I need something bigger for the shop here.